1. Field of the Invention
Heavy metals occur as cations in acidic (and, occasionally, alkaline) wastewaters produced during manufacture of printed circuit boards and in other industrial processes. Some wastewater streams containing copper cations are both highly concentrated (that is, in excess of several thousand ppm by weight of metal) and very acidic (pH below 1.0). Copper, lead and tin in particular may be recovered in saleable form from these wastewater streams by contacting them with scrap aluminum, another byproduct of the manufacture of PC boards, in an aqueous solution of sulfuric acid. Other metals of Groups IB, IIB, IIIA, and IVA of the Periodic Table also can be recovered using the process and apparatus of thin invention, to the extent cations of such metals may be present in wastewaters being treated. In the process of this invention, controlled, stepwise addition of aluminum during the reduction process, coupled with control of the acid concentration, improves reaction rates, reduces the danger of thermal runaway and limits oxidation of aluminum compared to conventional processes.
The invention relates to recovery of Group IB, IIB, IIIA and IVA metals from acidic or alkaline wastewaters produced in various manufacturing industries. More particularly, it relates to treatment of wastewater streams produced during PC board manufacturing operations to recover copper, lead and tin therefrom in saleable from, while reducing the concentration of toxic metal cations in the treated wastewater to levels safe for discharge to municipal sewer systems.
2. Description of the Prior Art
Plating of PC boards and subsequent steps in PC board manufacture produce some wastewater streams that contain very high concentrations of Cu.sup.+2 cations. These streams come from pickling acids and electroless copper solutions, and may contain from 1,000 ppm to as high as 500,000 Cu.sup.+2 by weight. They typically also contain high concentrations of H.sub.2 SO.sub.4, typically enough to produce pH levels of 1.0 or less. In some situations, such CU.sup.+2 -containing streams may be alkaline Additionally, washing steps that follow the etching process produce wastewater streams containing lower concentrations of Cu.sup.+2 cations, typically in the 20 ppm to 50 ppm range. Later washing steps produce wastewater streams containing Pb and Sn cations that result from subsequent soldering operations. Federal, state and local environmental regulations prohibit discharge of wastewaters containing Cu, Pb and Sn cations because of their toxicity; discharge levels of 1 ppm or below are considered desirable, and in some states such as California discharge levels as low as 0.4 ppm may be required. Discharge of highly acidic wastewaters to municipal sewers is also undesirable. Accordingly, various methods of reducing the cation content of these wastewaters and producing a near-neutral treated stream have been proposed.
A customary method of treating these wastewaters is by addition of large amounts of alkali salts, such as NaOH.multidot.Na.sub.2 CO.sub.3 or KOH, which causes precipitation of a floc of Cu(OH).sub.2. One such process is described in U.S. Pat. No. 3,816,306 (Roy). While this treatment also neutralizes the wastewater, it leaves Na+ or K+ cations in solution and produces large volumes of unsalable floc, essentially trading one disposal problem for another.
Another method of purification uses ion exchange to replace Cu.sup.+2 cations with less-toxic cations in solution. But ion exchange techniques also produce unsalable spent ion exchange media that must be disposed of in landfills or otherwise.
A more promising approach is to contact the wastewater containing metal cations including those of Cu, Pb or Sn with a more electropositive, non-toxic metal in elemental form. In this manner, the toxic cations are reduced to elemental state, a form in which they have value as byproducts, and the wastewater is cleansed of toxic metal cations. Because elemental aluminum is another byproduct of PC board manufacture, and aluminum is both low in toxicity and more electropositive than Cu, Pb or Sn, it is commonly suggested for recovery processes of this sort.
One such process is described in detail in U.S. Pat. No. 4,304,599 (Durkee). In that continuous process, wastewater containing Cu.sup.+2 cations is adjusted to a pH of 2 or less by addition of sulfuric acid, and then contacted in stagewise fashion with aluminum tailings retained between screens in a series of reaction cells through which the wastewater is pumped. Overall wastewater residence time is about three hours. A very large excess of aluminum is employed; in the example given by Durkee roughly 50 times the stoichiometric requirement for reaction with the Cu.sup.+2 cations being processed. (Even the first cell of the Durkee apparatus alone contains almost three times the stoichiometric requirement of elemental aluminum.) Following reaction with the aluminum, the wastewater contains about 50 ppm of Cu.sup.+2. It is then mixed with other waste streams that are substantially copper free, reducing the Cu.sup.+2 concentration to about 1.25 ppm. Finally, it is neutralized with caustic and discharged. Details of the reaction cell layout used for this process are described in U.S. Pat. No. 4,294,434 (Durkee).
In another version of the Durkee process, described in U.S. Pat. No. 4,450,002 (Durkee), certain modifications to the process allow operation at pH levels in the range of 3.2 to 3.3, thus reducing sulfuric acid requirements. This is achieved by adding a centrifuge to the discharge end of the series of reaction cells. The centrifuge removes very finely divided particles of copper which would otherwise escape in the effluent. The '002 patent also teaches that much slower reaction rates are achieved at the higher pH ranges used in this process, so that adequate conversion of Cu.sup.+2 may require recycle of part of the effluent stream back to the inlet of the series of reaction cells. As in the process described in the '599 patent, very large excesses of aluminum are used in comparison to stoichiometric requirements.
Yet another continuous process involving recycle of wastewater containing Cu.sup.+2 is described in U.S. Pat. No. 3,905,827 (Goffredo et al). In that process, continuous radial flow through a large fixed bed of aluminum turnings is used to purify rinsewater so that it can be reused over and over again in the PC board manufacturing process. As in both versions of the Durkee process, a large excess of aluminum is necessarily used. And like the Durkee process, the Goffredo process is not suitable for treating wastewater streams containing high concentrations of metal cations.
The Durkee processes recover saleable copper, but they have significant drawbacks. Use of aluminum in many times stoichiometric quantities is wasteful (scrap aluminum has substantial resale value itself). Undesired side reactions between the aluminum and the sulfuric acid increase the concentration of soluble Al.sup.+3 cations in the treated effluent. While not particularly toxic, such cations nevertheless are not desirable in treated water. In treating wastewater streams with high concentrations of Cu.sup.+2 cations, such as pickling acids and electroless copper solutions, use of large excesses of aluminum poses a danger of thermal runaway because of the exothermicity of the Cu - Al redox reaction. In fact, the Durkee process is designed for continuous processing of low-concentration wastewater streams, and will not operate satisfactorily for batch processing of wastewater streams containing high metal cation concentrations. (Although U.S. Pat. '002 to Durkee teaches application of the process to highly-concentrated wastewater streams (col. 10, lines 7-11), no specific examples are provided, and experimentation has shown that the Durkee process, with its large excess of aluminum, fails to operate satisfactory on high-concentration waste streams.) Moreover, the Durkee process actually reduces Cu.sup.+2 concentrations in the wastewater treated in the reaction cells only to levels around 50 ppm. In order to allow discharge to the sewer system at levels in the 1 ppm range, the treated wastewater is simply diluted with other wastewater that does not contain any copper. Federal and state environmental authorities generally will not accept wastewater treatment processes that can meet discharge concentration standards only by diluting an otherwise non-complying waste stream with clean water in order to reduce the concentration of pollutants to within acceptable limits. Thus, the Durkee process does not achieve substantially complete recovery of copper from the wastewater stream actually treated. The Durkee process does not provide for separate recovery of Pb and Sn from rinsewater streams that are also produced during other stages of PC board manufacture. Nor does Durkee provide for recovery of oxidized aluminum from the treated wastewater.
There is a need for a process and apparatus that safely achieves substantially complete copper recovery without need for final dilution, even when the feedstock contains very high levels of Cu.sup.+2 and/or other metal cations; that minimizes both the consumption of aluminum and the level of Al.sup.+3 cations in the treated effluent, and that can separately recover Pb, Sn and other metals of Groups IB, IIB, IIIA and IVA in saleable form from wastewater streams containing those contaminants.